Because of the structure of the turbomachine and/or its environment and/or its mounting sequence, it can happen that when it is necessary to fasten the outer ring of the bearing to the bearing support, access to one side of the assembly is no longer possible.
The assembly of the outer ring of the bearing and the flange to which it is mounted generally has the overall shape of a disk. Thus, the problematic situation in question is that in which it is possible to access the assembly from only one side of the disk, with access from the other side being impossible. The invention relates equally to the situation in which access is possible from the bearing support side and to the situation in which access is possible from the outer ring side.
Such a difficulty, or at least particularity, in the sequence of mounting a turbomachine shaft bearing arises in particular in the field of aviation, in certain turbomachines that are fitted with decoupling devices in their front portions. Such turbomachines are described in French patent FR 2 845 126.
FIG. 1 shows the arrangement of the front portion of such a turbomachine. In that turbomachine 6, a decoupling device is arranged at the front. That device serves to decouple the fan (not shown), which is connected to the low pressure shaft 10, from the remainder of the turbomachine for the purpose of limiting the damage that is caused by a failure occurring in the fan and causing unbalance to appear.
The fan is fastened by a flange 8 to the front of the low pressure shaft 10, which shaft is held so as to be rotatable about its axis X by a first bearing 12 behind the fan, and a second bearing 14 behind the first bearing (the front of the turbomachine 6 is on the left in FIG. 1).
The first bearing 12 is carried by a shroud 16 of generally conical shape that surrounds the shaft 10 and that extends rearwards from the first bearing 12 to a bearing support 20. The shroud 16 is fastened to the bearing support 20 by bolts 22.
The bearing support 20 is itself fastened to a stationary casing 24 of the turbomachine, mainly by means of bolts 26.
The bearing support 20 serves to fasten and hold in position not only the first bearing 12 (via the shroud 16), but also the second bearing 14. More precisely, the outer ring 30 of the second bearing 14 is fastened to the bearing support 20 by means of a set of bolt fasteners 40 of axes parallel to the axis X and regularly distributed around the circumferential of the outer ring 30.
In known manner, the outer ring 30 is of annular shape, surrounding and holding a set of rollers 31 on its radially outer side. On its radially inner side, the bearing 14 has an inner ring 33 that holds the rollers 31 on the inside.
The bearing support 20 comprises an outer annular flange 34 and an inner annular flange 36 connected together by spokes 48.
Since the shaft 10 having the fan fastened thereto is subjected to very large forces, the number of spokes 48 serving to hold the flange 36 relative to the flange 34 is quite high. Conversely, since the bearing support 20 is a part on board an airplane, its weight needs to be as light as possible. Thus, large passages 32 are arranged between the outer and inner flanges 34 and 36 of the bearing support 20 and between the spokes 48; these passages 32 are also essential for ventilating the enclosure of the turbomachine.
The outer flange 34 has two series of holes 38 and 42 that receive the above-mentioned bolts 26 and 22 respectively.
The inner flange 36 enables the outer ring 30 to be fastened, thereby holding the rotary shaft 10 at the bearing 14.
In order to fasten the ring 30, the flange 36 and the ring 30 have a series of identical through passages 44 through which the bolt fasteners 40 are passed in order to secure the bearing support 20 and the ring 30 rigidly together.
Mounting the bearing 14 of the turbomachine 6 presents the above-mentioned particularity: during such mounting, the front portion of the turbomachine (low pressure shaft 10, first bearing 12, shroud 16, bearing support 20) is assembled before the outer ring 30 is mounted. It follows that while the outer ring 30 is being mounted it is possible to access the bearing support 20 and the ring 30 from beside the ring only; conversely, at this stage, it is not possible to access the front side of the bearing support. It is therefore no longer possible to act on the front portions of the bolt fasteners 40, i.e. those portions that are on the front side; for example it is not possible to prevent the front portion of the bolt fastener 40 from turning in order to screw together the front portion of the bolt fastener and its rear portion.
The technical solution used in the prior art for remedying this difficulty can be seen in FIGS. 1 and 2. It consists in using bolt fasteners 40 comprising two portions, namely a captive nut 50 (front portion) and a fastener bolt 46 (rear portion). The bolt fastener may also include secondary parts such as washers, etc.
The rear portion of the fastener 40 is a fastener bolt 46 having a head at its rear end and a shank extending forwards, with the end of the shank being threaded. This fastener bolt 46 is designed to act as a fuse in shear, should it be necessary to actuate the decoupling device and release the shroud 16 from its fastening to the bearing support 20.
Furthermore, the front portion of the fastener 40 is a captive nut 50 that is fixed to a collar 58 provided on the bearing support 20.
The nut 50 is a standard nut and it is said to be “captive” because it is prevented from turning, or held “captive”, by a nut support 54 made of sheet metal that holds it in position. The nut support 54 enables the nut 50 to be fixed to the bearing support and enables it to be held in position until the moment when it is finally fastened as a result of being screwed onto the bolt 46. During this screw fastening operation, the nut support 54 also ensures that the nut 50 does not turn while the bolt 46 is being screwed into it.
In order to keep the nut 50 in position, the nut support 54 is itself fixed to the bearing support 20. This operation of fixing the nut 50 is performed before the bearing support 20 is mounted on the front portion of the turbomachine 6, at a time when the front side of the bearing support 20 is still accessible. In order to avoid any turning of the captive nut 50 before the bolts 46 are screwed into them, the supports 54 are fixed by means of two rivets (a single rivet would not suffice).
In that prior system, the threads in the nuts 50 are located inside them: as a result there is little risk of damaging those threads in the period between mounting the nuts 50 and subsequently screwing the bolts 46 into them.
It should also be observed that the nut 50 is held by the support 54 with a small amount of clearance, perpendicularly to the axis X. This clearance is necessary. While the ring 30 is being mounted on the bearing support, the ring 30 is initially put into an almost final position by putting it into place using the rollers 31. Thereafter, the ring is fastened by putting the various bolts 46 into place and screwing them into the corresponding nuts 50. When the bolts 46 are caused to penetrate into the passages 44 provided in the bearing support, the ring 30 takes up its final position, the passages 44 being provided with just sufficient clearance to enable the bolts 46 to pass through them.
When the bolts 46 begin to be screwed into the nuts 50, the nuts shift a little so that their respective axes coincide with the axes of the bolts. This adjustment of the positions of the nut axes is made possible by the above-mentioned small clearance between the nuts and the supports 54.
The above-described arrangement of the fasteners 40 enables the outer ring 30 to be mounted but it presents various drawbacks.
In particular, the collar 58 is formed on the front of the bearing support 20. The nuts 50 are fixed to the front of the bearing support, in front of the spokes 48. As a result, the bearing support 20 and the ring 30 are pierced over a great length by the through passages 44. The presence of these passages makes it necessary to provide bulges in the spokes 48: the spokes are thus of swollen shape along the passages 44 in order to be sufficiently strong.
The length of the bolts 46 together with the amount of swelling made necessary by the passages 44 result in the fastening of the ring 30 on the support 20 presenting a weight that is not negligible.
Furthermore, the prior mounting of nuts on the bearing support is an operation that is relatively lengthy because of the need to fasten two rivets for each nut.